1. Field of the Invention
The present invention is related to wave plates and, more particularly, to wave plates including uniaxial crystals for achieving an insensitivity to angle of incidence.
2. Description of Related Art
Wave plates are devices that alter the polarization of optical signals when optical signals pass through the wave plate. Birefringent materials, which are refractive materials typically used for wave plates, exhibit an anisotropic index of refraction or velocity of propagation of optical signals passing therethrough. The index of refraction varies for different polarizations, and the relative amount of phase shift of the polarized optical signals through the birefringent material is generally referred to as retardance. At any specific wavelength, the phase difference is governed by the thickness of the wave plate.
Birefringent materials separate incoming signals into orthogonally polarized beams that propagate with their polarization directions along different axes, namely extraordinary and ordinary axes. Uniaxial birefringent materials include a single axis (i.e., extraordinary axis) that defines a direction along which signals propagate differently than along the remaining two axes (i.e., ordinary axes). Some uniaxial birefringent materials, such as quartz, include a symmetry axis (i.e., optic axis) having two principal indices of refraction: a slow axis and a fast axis. The axis along which the material has the smallest refractive index is the fast axis, such that optical signals polarized parallel to the fast axis travel faster than optical signals parallel to the slow axis. With respect to positive uniaxial crystals, such as quartz, the extraordinary axis is the slow axis, while the ordinary axis is the fast axis. In contrast, the extraordinary axis of negative uniaxial crystals, such as sapphire, is the fast axis, while the ordinary axis is the slow axis.
The design of optical systems that employ polarization wave plates can be unduly limited by changes in wave plate performance with angle of incidence of the incoming signals. In general, systems employing a wave plate require the retardance to be constant independent of angle, otherwise system performance is degraded as the angle of the optical signal changes. However, the amount of retardation of a conventional wave plate changes as the angle of the optical signals passing through it changes. This effect can sometimes be made small by using a true zero-order wave plate, but these wave plates are extremely thin and delicate if made of conventional birefringent crystals. Multi-order or compound pseudo zero-order wave plates are made of crystals with more reasonable (practical) thicknesses, but the range of angles they can be used over is undesirably limited by the angular dependence of their retardance.
It would therefore be advantageous to provide an improved wave plate capable of compensating for angular sensitivity. In addition, it would be advantageous to provide a wave plate capable of achieving a desired retardance. Furthermore, it would be advantageous to provide a wave plate that is durable and practical for various applications over a greater range of angles.